The present invention relates to a transmission device and a transmission method, and particularly to a transmission device and a transmission method which are suitably used to transmit digital audio signals reproduced, for example, by a digital audio instrument to a speaker, a headphone, a recording instrument, etc., with infrared ray.
When audio signals reproduced by an audio instrument -such as a CD (compact disc) reproducing device or the like are transmitted to another audio instrument such as a speaker, a headphone or the like, it is generally required to connect these instruments through a physical transmission line such as an audio cable or the like. However, with respect to the speaker, etc., sufficient stereophonic effect cannot be obtained unless the speaker is disposed far away from the audio instrument for reproducing audio signals. Therefore, an extremely long cable is needed as the audio cable for connecting the CD reproducing device and the speaker.
In order to obtain a sufficient stereophonic effect, the setup position of the speak is an important factor. However, when an audio cable is used, the setup position is restricted by the length of the audio cable.
Further, when a headphone is used through an audio cable, a user's action is restricted by the audio cable.
Therefore, there has been recently used an audio signal transmission device for transmitting audio signals reproduced by an audio instrument such as a CD reproducing device to another audio instrument such as a speaker, a headphone or the like with infrared ray. In this audio signal transmission device, audio signals to be transmitted are subjected to frequency modulation at a CD reproducing device side, and a light emitter such as a infrared ray emitter is driven on the basis of the modulation signals thus obtained, whereby the infrared ray corresponding to the modulation signals is emitted. The infrared ray propagates in the air, and is received by an infrared ray detector at the reception side such as the speaker or the headphone, for example. At the reception side, the received infrared ray is converted to the reception signals corresponding to the modulation signals, and further the reception signals are demodulated to reproduce the original audio signals.
Thus, according to the above-described audio signal transmission device, the audio signals can be transmitted collectively to many audio instruments with no physical transmission line.
However, in such an audio signal transmission device as described above, the audio signals are subjected to analog modulation such as frequency modulation or the like, and thus when the distance between audio instruments between the audio signals are transmitted is long, there is a problem that the S/N of the audio signals at the reception side is greatly deteriorated.
Therefore, in order to enhance sound quality, there has been recently proposed a digital system for digitally modulating the digital audio signals and then transmitting the signals.
When such a digital system is used, from the viewpoint of the transmission efficiency, it is ideal that the ratio of the data rate of the digital audio signals and the transmission rate thereof be equal to 1. However, for the transmission of the digital audio signals, as it is necessary to transmit an error correcting code and other necessary data, and actually, the transmission rate is higher than the data rate.
Accordingly, in order to enhance the transmission efficiency, it is best to approach the ratio of the transmission rate and the data rate to 1, however, when the ratio is complicated, the data processing in the modulation and demodulation processes is complicated.
Therefore, the transmission rate must be determined while the enhancement of the transmission efficiency and the complication of the data processing are balanced with each other.
Further, the frequency band which can be used to transmit audio signals with infrared ray is provided in the standard CP-1205 of EIAJ (Electronic Industries Association of Japan) and IEC (International Electrotechnical Commission) 1603, and the transmission rate must be determined so that the transmission band used for the transmission does not exceed the frequency band as described above.
Moreover, it is necessary to provide a transmission system for audio signals with infrared ray, which meets users' needs.